Proximity detector employing magneto resistive sensor in the central magnetic field null of a toroidal magnet

Abstract

The present invention is a proximity sensor employing a magneto resistive structure. This proximity sensor includes a toroidal magnet having a central aperture, an inner circumference and an outer circumference. This toroidal magnet has a first magnetic pole at the inner circumference and a second magnetic pole at the outer circumference thereby creating a magnetic field null at a center portion of the central aperture. A magneto resistive structure is disposed at the magnetic field null in the central aperture of the toroidal magnetic. This magneto resistive structure has a resistance dependent upon magnetic flux. The proximity of a magnetically permeable target to the toroidal magnet shifts the position of the magnetic null, thereby causing a large change in the magnetic flux through the magneto resistive structure. An electrical detection circuit detects changes in this resistance for indication of the proximity of the magnetically permeable target.

We claim: 
     
       1. A proximity sensor comprising: a toroidal magnet consisting of a disk of permanent magnetic material having an outer circumference and a circular central aperture therethrough forming an inner circumference, and a band of material having a high magnetic permeability disposed at said outer circumference, said disk of permanent magnetic material constructed with a first magnetic pole at said inner circumference and a second magnetic pole at said outer circumference thereby creating a magnetic null at a center position of said central aperture; a magneto resistive structure disposed at said central portion of said central aperture of said toroidal magnet, thereby disposed at said magnetic null, having an electrical resistance proportional to the magnetic flux therethrough from said toroidal magnet; and   an electrical detection circuit connected to said magneto resistive structure for detecting changes in said electrical resistance for indication of the proximity of a magnetically permeable target to said proximity sensor.   
     
     
       2. The proximity sensor claimed in claim 1, wherein: said band of material having a high magnetic permeability includes steel.   
     
     
       3. The proximity sensor claimed in claim 1, wherein: said electrical detection circuit includes a resistance measurement circuit connected to said magneto resistive structure for generating an electrical output signal proportional to the resistance of said magneto resistive structure and a comparator connected to said resistance measurement circuit for comparing said electrical output signal to a predetermined reference signal for generating said indication of the proximity of a magnetic target to said proximity sensor when said electrical output signal exceeds said predetermined reference signal.   
     
     
       4. The proximity sensor claimed in claim 3, wherein: said predetermined reference signal of said comparator is operator adjustable thereby permitting adjustment of the sensitivity of said proximity detector.   
     
     
       5. The proximity sensor claimed in claim 3, wherein: said electrical detection circuit further includes a triangle wave output circuit connected to said comparator for generating a triangle wave output signal having an up sloping output of predetermined slope when said comparator generates said indication of the proximity of a magnetic target to said proximity sensor and having a down sloping output of predetermined slope when said comparator does not generate said indication of the proximity of a magnetic target to said proximity sensor.   
     
     
       6. The proximity sensor claimed in claim 5, wherein: said electrical detection circuit further includes a square wave output circuit connected to said triangle wave output circuit for generating a square wave output signal having a first level when said triangle wave output signal is greater than a predetermined level and having a second level when said triangle wave output signal is less than said predetermined level.   
     
     
       7. A proximity sensor comprising: a toroidal magnet consisting of a first circular band of material having a high magnetic permeability disposed at an inner circumference forming a circular central aperture, a second circular band of material having a high magnetic permeability disposed at an outer circumference concentric with said inner circumference, and a plurality of spokes of permanent magnetic material disposed between said inner circumference and said outer circumference, each spoke of permanent magnetic material having a first magnetic pole disposed at said at said inner circumference and a second magnetic pole disposed at said outer circumference thereby creating a magnetic null at a center position of said central aperture;   a magneto resistive structure disposed at said central portion of said central aperture of said toroidal magnet, thereby disposed at said magnetic null, having an electrical resistance proportional to the magnetic flux therethrough from said toroidal magnet; and   an electrical detection circuit connected to said magneto resistive structure for detecting changes in said electrical resistance for indication of the proximity of a magnetically permeable target to said proximity sensor.   
     
     
       8. The proximity sensor claimed in claim 7, wherein: said first and second bands of material having a high magnetic permeability includes steel.   
     
     
       9. The proximity sensor claimed in claim 7, wherein: said plurality of spokes of permanent magnetic material consists of four spokes of permanent magnetic material disposed at ninety degree intervals about said inner circumference.   
     
     
       10. The proximity sensor claimed in claim 7, wherein: said electrical detection circuit includes a resistance measurement circuit connected to said magneto resistive structure for generating an electrical output signal proportional to the resistance of said magneto resistive structure and a comparator connected to said resistance measurement circuit for comparing said electrical output signal to a predetermined reference signal for generating said indication of the proximity of a magnetic target to said proximity sensor when said electrical output signal exceeds said predetermined reference signal.   
     
     
       11. The proximity sensor claimed in claim 10, wherein: said predetermined reference signal of said comparator is operator adjustable thereby permitting adjustment of the sensitivity of said proximity detector.   
     
     
       12. The proximity sensor claimed in claim 11, wherein: said electrical detection circuit further includes a triangle wave output circuit connected to said comparator for generating a triangle wave output signal having an up sloping output of predetermined slope when said comparator generates said indication of the proximity of a magnetic target to said proximity sensor and having a down sloping output of predetermined slope when said comparator does not generate said indication of the proximity of a magnetic target to said proximity sensor.   
     
     
       13. The proximity sensor claimed in claim 12, wherein: said electrical detection circuit further includes a square wave output circuit connected to said triangle wave output circuit for generating a square wave output signal having a first level when said triangle wave output signal is greater than a predetermined level and having a second level when said triangle wave output signal is less than said predetermined level.   
     
     
       14. A proximity sensor comprising: a toroidal magnet having a central aperture, an inner circumference and an outer circumference with a first magnetic pole at said inner circumference and a second magnetic pole at said outer circumference thereby creating a magnetic null at a center position of said central aperture; and   a magneto resistive structure disposed at said center position of said central aperture of said toroidal magnetic having an electrical resistance proportional to the magnetic flux therethrough from said toroidal magnet resulting from a shift in the position of said magnetic null caused by the proximity of a magnetically permeable target to said toroidal magnet.